Laser welding is commonly used to join plastic or resinous parts, such as thermoplastic parts, at a welding zone. An example of such use of lasers can be found in U.S. Pat. No. 4,636,609, which is expressly incorporated herein by reference.
As is well known, lasers provide a semi-focused beam of electromagnetic radiation at a specified frequency (i.e., coherent monochromatic radiation). There are a number of types of lasers available; however, infrared lasers or non-coherent sources provide a relatively economical source of radiative energy for use in heating a welding zone. One particular example of infrared welding is known as Through-Transmission Infrared (TTIr) Welding. TTIr welding employs an infrared laser capable of producing infrared radiation that is directed by lenses, diffractive optics, fiber optics, waveguides, lightpipes, or lightguides through a first plastic part and into a second plastic part. This first plastic part is often referred to as the transmissive piece, since it generally permits the laser beam from the laser to pass therethrough. However, the second plastic part is often referred to as absorptive piece, since this piece (and/or an absorptive additive at the weld interface) generally absorbs the radiative energy of the laser beam to produce heat in the welding zone. This heat in the welding zone causes the transmissive piece and the absorptive piece to be melted and, with intimate contact, welded together.
With reference to FIGS. 1A and 1B, typical through transmission infrared (TTIr) systems 100 and 100′ for laser welding of plastics are shown. A beam of infrared laser light 102 from a source of infrared laser light 104 is directed to the plastic parts 106, 108 to be welded. The infrared laser light passes through transmissive plastic part 106 to a weld interface 110 at a junction of transmissive plastic part and absorptive plastic part 108. Weld interface 110 is also sometimes referred to in the art as a weld site, a weld region or a weld area. An infrared absorber additive 112 may be provided at weld interface 110 (FIG. 1A) The absorption of the laser light heats up the weld interface at the junction of the parts 106, 108, melting the plastic in both parts 106, 108 at the weld interface 110. The laser light is removed, such as by turning laser source 102 off, after an appropriate period of time and the molten plastic at weld interface 110 then cools, thus welding the two plastic parts 106, 108 together.
Oftentimes, the absorptive second plastic part 108, or the infrared absorber additive 112 used at the weld interface 110, are relatively low absorbers of the infrared light. A large portion, indicated at 114, of the infrared laser light 102 then passes though both parts 106, 108 and out of part 108, becoming wasted in the process.
With low absorbers, either too low a laser energy is delivered to the weld interface 110 to make a weld, or relatively high laser energies need to be used to translate into enough energy at the weld interface 110 to make a weld.